1. Field of the Invention
The present invention relates to a liquid crystal display device and, specifically to a liquid crystal display device having improved front surface luminance.
2. Description of the Related Art
A liquid crystal display device (hereinafter, also referred to as LCD) has been used in a broadening range of fields every year as an image display device which has low power consumption and saves spaces. A liquid crystal display device has a constitution in which a backlight (hereinafter, also referred to as BL), a backlight-side polarizing plate, a liquid crystal cell, a display-side polarizing plate, and the like are provided in this order.
Recently, for liquid crystal display devices, development for power saving, high definition, and color reproducibility improvement has been underway in order to improve LCD performance. Hitherto, while there has been a significant demand for power saving, high definition, and color reproducibility improvement in small-sized display devices such as tablet PCs and smartphones, development for high definition (4K2K) and high color reproduction region (European Broadcasting Union (EBU) ratio of 100% or higher) are underway in large-sized display devices for next-generation displays with current TV standards (FHD, 72% of National Television System Committee (NTSC) ratio≈100% of EBU ratio), and there is an intensifying demand for power saving, high definition, and color reproducibility improvement in liquid crystal display devices.
In accordance with power saving in the backlight, there are cases in which an optical sheet member is provided between the backlight and the backlight-side polarizing plate in order to increase the light utilization efficiency. The optical sheet member is an optical element that transmits only light rays vibrating in a specific polarization direction among incidence light rays vibrating in random directions and reflects light rays vibrating in other polarization directions. As a core component of a low-power LCD developed in response to an increase in mobile devices and a decrease in the power consumption of home appliance, it is expected to increase luminance (the degree of brightness of a light source per unit area) by solving the low light utilization efficiency of LCDs.
As the above-described optical sheet member, a technique is known in which a specific optical sheet member (Dual Brightness Enhancement Film (registered trademark, DBEF) or the like) is provided between the backlight and the backlight-side polarizing plate so as to improve the light utilization ratio of the BL by means of light recycling and thus the luminance of the BL is improved while saving power in the backlight (refer to JP3448626B). Similarly, JP1989-133003A (JP-H1-133003A) describes a polarizing plate obtained by laminating a λ/4 plate and a cholesteric liquid crystalline phase. When the bandwidth is broadened in a layer formed by fixing three or more layers of a cholesteric liquid crystalline phase having different pitches between the cholesteric liquid crystalline phases, it is possible to improve the light utilization ratio of the BL by means of light recycling.
However, the above-described optical sheet member has a complicated member constitution, and thus, in order to distribute the optical sheet member in the market, it becomes essential to reduce the cost by decreasing the number of members by means of additional integration of functions of the members.
From the viewpoint of high definition and color reproducibility improvement of a liquid crystal display device, a method for sharpening the light emission spectrum of the backlight is also known. For example, JP2012-169271A describes a method for increasing luminance and improving color reproducibility by realizing white light using a quantum dot (QD) which emits red light and green light as a fluorescent body between a blue LED and a light guide plate. In SID'12 DIGEST p. 895, a method of combining a light conversion sheet (QDEF, also referred to as quantum dot sheet) in which a quantum dot is used in order to improve the color reproducibility of the LCD is proposed.
In addition, JP2012-22028A proposes a method for decreasing the energy loss in a color filter and increasing the energy efficiency of a liquid crystal display by disposing a fluorescent layer including a fluorescent body made up of quantum dots on a path of light radiated from a violet LED or a blue LED.
Meanwhile, in JP2012-502322A, a liquid crystal display device is proposed in which a blue light source, a cholesteric liquid crystal, a light conversion layer capable of altering the wavelength of light to a longer value, and a λ/4 plate are combined together, thereby providing clearly visible bright images under a bright ambient light condition with a low power consumption and improving long-term reliability.
As a light conversion material in which the above-described quantum dot or the like is used, recently, a compound-based circularly polarized luminescence fluorescent material also has been known (refer to JP2009-242501A). However, JP2009-242501A does not describe any examples in which the compound-based circularly-polarized light-emitting fluorescent material is actually used for a liquid crystal display device.